Compressed gas released into a gaseous ambience through a nozzle or orifice forms a fast moving jet which quickly aspirates ambient gases and becomes diluted. Aspiration of air or other gases present in the jet environment is observed in the thermal spray-coating industry, industrial combustion heating and melting, oxygen lancing in steelmaking, as well as various thermal management, welding, pumping and painting applications. The extent of aspiration becomes significant for turbulent gas jets characterized by high Reynolds numbers. The results of aspiration can be detrimental or beneficial, depending on application and process requirements. In both cases, however, there is a need to develop an effective method and apparatus to improve control of gas aspirated into a turbulent gas jet.
Aspiration of ambient air poses a very difficult problem in thermal spray-coating operations involving supersonic and subsonic hot jets of relatively inert or reducing gases carrying reactive droplets or particles of metallic or ceramic feed materials which subsequently form coatings or deposits on an impacted surface. In such thermal spray-coating operations, air aspiration results in oxidation of the coating in a manner which can be very detrimental. In order to address this problem, various new designs of plasma, combustion, and electric arc spraying guns have been proposed as have special retrofit attachments for commercially available spraying guns. In general, such attempts have fallen short because they failed to establish criteria for aspiration flowrates which result from the broad range of turbulent gas jets encountered in the industry. Also, many of the proposed design modifications interfered with the flow field of jets produced by the original equipment.
Among the more common proposals to deal with this problem have been structural and external gas shrouding devices, many of which have proven to be impracticable because either they were too large and required too short of a standoff distance for typical shop conditions or they offered only marginal improvement. Although the history of gas shrouding spans over three decades, the problems involved in protecting and modifying gas jets still cry for solutions that have not been forthcoming.
In an early reference on gas shrouding, Arata et al., U.S. Pat. No. 3,082,314 (1963) describe a plasma arc torch for cutting or welding having a concentric gas shield to reduce electrode erosion and control temperature. Somewhat later and more relevant to the situations discussed above, Jackson, U.S. Pat. No. 3,470,347 (1969) deals with the problem of keeping oxygen away from a coating applied to a substrate with a plasma arc torch. This is said to be accomplished by protecting the torch gas effluent by surrounding it with a forward flowing coaxial annular shield of gas having a width and flow rate corresponding by formula to the torch orifice diameter. Although it is stated that the arc amperage and arc gas flow rate have a negligible effect on the shielding effectiveness, as a practical matter from the information supplied, it is not possible to scale up the operation or adapt it to different types of plasma, arc-wire or combustion spraying guns and burners.
Guest et al., U.S. Pat. No. 3,892,882 (1975) describe a plasma spraying operation in which a zone of sub-atmospheric pressure is maintained through which the spray jet and entrained coating powder pass on the way from the nozzle to the work piece. The sub-atmospheric pressure can be produced by injecting a sheath of gas moving in a spiral path along the inner surface of a tube surrounding the jet spray path, or by a vacuum pump. The disclosed long shielding tubes are impractical in many robotics and manual spray-coating operations that can accept only compact or recessed attachments to the gun nozzle and are unacceptable for burners jetting flames into high temperature furnace chambers.
Smyth, U.S. Pat. No. 4,121,083 (1978) describes a plasma jet spraying device having positioned around the jet opening a wall shroud within which a gaseous flame shroud is formed. This gas shroud is introduced either at an angle to the jet flow or countercurrent or concurrent to the jet flow.
Browning, U.S. Pat. No. 4,634,611 (1987) describes a flame spraying device having the main jet spray shrouded with warm high velocity air in order to increase the velocity of the jet spray beyond the nozzle. Such an air sheath would increase aspiration of oxygen into the jet stream, not reduce it, and, therefore, be counterproductive to the desired protection of an applied coating from oxidation.
Moskowitz, U.S. Pat. No. 4,869,936 (1989) describes a metal shielding attachment for supersonic thermal spray equipment which tangentially introduces a shield gas in a shroud surrounding the gas jet so that the shielding gas has a helical flow path all the way to the work piece. This is intended to address the problem of oxidation of the coating. The attachment uses shield gas nozzles arranged in a circular array adjacent to the jet orifice to inject shield gas tangentially against the inner wall of the shroud, which can be a double walled structure to permit circulation of cooling water within it. This device suffers from the same disadvantages as the apparatus of Guest described in the '882 patent.
More recently, Reiter, U.S. Pat. No. 5,154,354 (1992) discloses what is apparently intended to be an improvement on the device of the '347 patent to Jackson in order to reduce eddying and penetration of the gas shield by surrounding air. This is done by placing a protective gas nozzle with a core hollow space around the spray jet nozzle. The protective gas flow is directed concurrently with the spray jet in a manner said to be free of eddy currents. Although the description of the device is obscure, it is clear that the intent is to accelerate the protective gas mantle as it is introduced around the jet spray. In practice, such devices have fallen short of their objectives.